Techniques to improve polyurethane membranes for implantable glucose sensors

ABSTRACT

The invention provides an implantable membrane for regulating the transport of analytes therethrough that includes a matrix including a first polymer; and a second polymer dispersed throughout the matrix, wherein the second polymer forms a network of microdomains which when hydrated are not observable using photomicroscopy at 400× magnification or less. In one aspect, the homogeneous membrane of the present invention has hydrophilic domains dispersed substantially throughout a hydrophobic matrix to provide an optimum balance between oxygen and glucose transport to an electrochemical glucose sensor.

RELATED APPLICATION

This application is a division of U.S. application Ser. No. 10/153,356filed May 22, 2002 now U.S. Pat. No. 7,226,978, the disclosure of whichis hereby incorporated by reference in its entirety and is made aportion of this application.

FIELD OF THE INVENTION

The present invention relates generally to membranes for use incombination with implantable devices for evaluating an analyte in a bodyfluid. More particularly, the invention relates to membranes forcontrolling the diffusion of glucose therethrough to a glucose sensor.

BACKGROUND OF THE INVENTION

A biosensor is a device that uses biological recognition properties forthe selective analysis of various analytes or biomolecules. Generally,the sensor will produce a signal that is quantitatively related to theconcentration of the analyte. In particular, a great deal of researchhas been directed toward the development of a glucose sensor that wouldfunction in vivo to monitor a patient's blood glucose level. Such aglucose sensor is useful in the treatment of diabetes mellitus. Inparticular, an implantable glucose sensor that would continuouslymonitor the patient's blood glucose level would provide a physician withmore accurate information in order to develop optimal therapy. One typeof glucose sensor is the amperometric electrochemical glucose sensor.Typically, an electrochemical glucose sensor employs the use of aglucose oxidase enzyme to catalyze the reaction between glucose andoxygen and subsequently generate an electrical signal. The reactioncatalyzed by glucose oxidase yields gluconic acid and hydrogen peroxideas shown in the reaction below (equation 1):

${glucose} + {O_{2}\underset{oxidase}{\overset{glucose}{\rightarrow}}{gluconic}\mspace{14mu}{acid}} + {H_{2}O_{2}}$The hydrogen peroxide reacts electrochemically as shown below inequation 2:H₂O₂→2H⁺+O₂+2e ⁻

The current measured by the sensor is generated by the oxidation of thehydrogen peroxide at a platinum working electrode. According to equation1, if there is excess oxygen for equation 1, then the hydrogen peroxideis stoichiometrically related to the amount of glucose that reacts withthe enzyme. In this instance, the ultimate current is also proportionalto the amount of glucose that reacts with the enzyme. However, if thereis insufficient oxygen for all of the glucose to react with the enzyme,then the current will be proportional to the oxygen concentration, notthe glucose concentration. For the glucose sensor to be useful, glucosemust be the limiting reagent, i.e., the oxygen concentration must be inexcess for all potential glucose concentrations. Unfortunately, thisrequirement is not easily achieved. For example, in the subcutaneoustissue the concentration of oxygen is much less that of glucose. As aconsequence, oxygen can become a limiting reactant, giving rise to aproblem with oxygen deficit. Attempts have been made to circumvent thisproblem in order to allow the sensor to continuously operate in anenvironment with an excess of oxygen.

Several attempts have been made to use membranes of various types in aneffort to design a membrane that regulates the transport of oxygen andglucose to the sensing elements of glucose oxidase-based glucosesensors. One approach has been to develop homogenous membranes havinghydrophilic domains dispersed substantially throughout a hydrophobicmatrix to circumvent the oxygen deficit problem, where glucose diffusionis facilitated by the hydrophilic segments.

For example, U.S. Pat. No. 5,322,063 to Allen et al. teaches thatvarious compositions of hydrophilic polyurethanes can be used in orderto control the ratios of the diffusion coefficients of oxygen to glucosein an implantable glucose sensor. In particular, various polyurethanecompositions were synthesized that were capable of absorbing from 10 to50% of their dry weight of water. The polyurethanes were renderedhydrophilic by incorporating polyethyleneoxide as their soft segmentdiols. One disadvantage of this invention is that the primary backbonestructure of the polyurethane is sufficiently different so that morethan one casting solvent may be required to fabricate the membranes.This reduces the ease with which the membranes may be manufactured andmay further reduce the reproducibility of the membrane. Furthermore,neither the percent of the polyethyleneoxide soft segment nor thepercent water pickup of the polyurethanes disclosed by Allen directlycorrelate to the oxygen to glucose permeability ratios. Therefore, oneskilled in the art cannot simply change the polymer composition and beable to predict the oxygen to glucose permeability ratios. As a result,a large number of polymers would need to be synthesized before a desiredspecific oxygen to glucose permeability ratio could be obtained.

U.S. Pat. Nos. 5,777,060 and 5,882,494, each to Van Antwerp, alsodisclose homogeneous membranes having hydrophilic domains dispersedthroughout a hydrophobic matrix to reduce the amount of glucosediffusion to the working electrode of a biosensor. For example, U.S.Pat. No. 5,882,494 to Van Antwerp discloses a membrane including thereaction products of a diisocyanate, a hydrophilic diol or diamine, anda silicone material. In addition, U.S. Pat. No. 5,777,060 to Van Antwerpdiscloses polymeric membranes that can be prepared from (a) adiisocyanate, (b) a hydrophilic polymer, (c) a siloxane polymer havingfunctional groups at the chain termini, and optionally (d) a chainextender. Polymerization of these membranes typically requires heatingof the reaction mixture for periods of time from 1 to 4 hours, dependingon whether polymerization of the reactants is carried out in bulk or ina solvent system. Therefore, it would be beneficial to provide a methodof preparing a homogenous membrane from commercial polymers. Moreover,as mentioned above, one skilled in the art cannot simply change thepolymer composition and be able to predict the oxygen to glucosepermeability ratios. Therefore, a large number of polymers would need tobe synthesized and coating or casting techniques optimized before adesired specific oxygen to glucose permeability ratio could be obtained.

A further membrane is disclosed in U.S. Pat. No. 6,200,772 B1 to Vadgamaet al. that has hydrophilic domains dispersed substantially throughout ahydrophobic matrix for limiting the amount of glucose diffusing to aworking electrode. In particular, the patent describes a sensor devicethat includes a membrane comprised of modified polyurethane that issubstantially non-porous and incorporates a non-ionic surfactant as amodifier. The non-ionic surfactant is disclosed as preferably includinga poly-oxyalkylene chain, such as one derived from multiple units ofpoly-oxyethylene groups. As described, the non-ionic surfactant may beincorporated into the polyurethane by admixture or through compoundingto distribute it throughout the polyurethane. The non-ionic surfactantis, according to the specification, preferably incorporated into thepolyurethane by allowing it to react chemically with the polyurethane sothat it becomes chemically bound into its molecular structure. Like mostreactive polymer resins, complete reaction of the surfactant into thepolyurethane may never occur. Therefore, a disadvantage of this membraneis that it can leach the surfactant over time and cause irritation atthe implant site or change its permeability to glucose.

PCT Application WO 92/13271 discloses an implantable fluid measuringdevice for determining the presence and the amounts of substances in abiological fluid that includes a membrane for limiting the amount of asubstance that passes therethrough. In particular, this applicationdiscloses a membrane including a blend of two substantially similarpolyurethane urea copolymers, one having a glucose permeability that issomewhat higher than preferred and the other having a glucosepermeability that is somewhat lower than preferred.

An important factor in obtaining a useful implantable sensor fordetection of glucose or other analytes is the need for optimization ofmaterials and methods in order to obtain predictable in vitro and invivo function. The ability of the sensor to function in a predictableand reliable manner in vitro is dependent on consistent fabricationtechniques. Repeatability of fabrication has been a problem associatedwith prior art membranes that attempt to regulate the transport ofanalytes to the sensing elements.

We refer now to FIG. 1, which shows a photomicrograph at 200×magnification of a prior art cast polymer blend following hydration. Adisadvantage of the prior art membranes is that, upon thermodynamicseparation from the hydrophobic portions, the hydrophilic componentsform undesirable structures that appear circular 1 and elliptical 2 whenviewed with a light microscope when the membrane 3 is hydrated, but notwhen it is dry. These hydrated structures can be detected byphotomicroscopy under magnifications in the range of between 200×-400×,for example. They have been shown by the present inventors to benon-uniform in their dimensions throughout the membrane, with some beingof the same size and same order of dimensions as the electrode size. Itis believed that these large domains present a problem in that theyresult in a locally high concentration of either hydrophobic orhydrophilic material in association with the electrode. This can resultin glucose diffusion being limited or variable across the dimensionadjacent the sensing electrode. Moreover, these large hydratedstructures can severely limit the number of glucose diffusion pathsavailable. It is noted that particles 4 in membrane 3 are dustparticles.

With reference now to a schematic representation of a known membrane 14in FIG. 2A, one can consider by way of example a continuous path 16 bywhich glucose may traverse along the hydrophilic segments 10 that aredispersed in hydrophobic sections 12 of the membrane. For path 16,glucose is able to traverse a fairly continuous path along assembledhydrophilic segments 10 from the side 18 of the membrane in contact withthe body fluid containing glucose to the sensing side 20 proximal tosensor 22, where an electrode 24 is placed at position 26 where glucosediffusion occurs adjacent surface 20. In particular, in that portion ofthe membrane 14 proximal to position 26, glucose diffusion occurs alonghydrophilic segments 10 that comprise a hydrated structure 28 having asize and overall dimensions x that are of the same order of magnitude aselectrode 24. Therefore, glucose diffusion would be substantiallyconstant across the dimension adjacent electrode 24, but the number ofglucose diffusion paths would be limited.

Referring now to FIG. 2B, one can consider an example where glucosetraversing prior art membrane 14 from side 18 in contact with the bodyfluid to the sensing side 20 cannot adequately reach electrode 30. Inparticular, electrode 30 is located at position 34, which is adjacent toa locally high concentration of a hydrophobic region 12 of prior artmembrane 14. In this instance, glucose diffusion cannot adequatelyoccur, or is severely limited across the dimension adjacent theelectrode surface. Consequently, one would expect that the locally highconcentration of the hydrophobic regions adjacent to working electrode30 would limit the ability of the sensing device to obtain accurateglucose measurements. The random chance that the membrane could beplaced in the 2A configuration as opposed to 2B leads to widevariability in sensor performance.

We also refer to FIG. 2C, which shows another cross-section of prior artmembrane 14. In this instance, glucose is able to traverse a fairlycontinuous path 36 from side 18 to side 20 proximal to the sensingdevice. However, electrode 38 is located at position 40 such thatglucose diffusion is variable across the dimension adjacent theelectrical surface. In particular, most of the electrode surface isassociated with a locally high concentration of hydrophobic region and asmall portion is associated with hydrophilic segments 10 along glucosediffusion path 36. Furthermore, glucose diffusing along path 36 a wouldnot be associated with the electrode. Again, the large non-uniformstructures of the prior art membranes can limit the number of glucosediffusion paths and the ability of the sensing device to obtain accurateglucose measurements.

It would be beneficial to form more homogeneous membranes forcontrolling glucose transport from commercially available polymers thathave a similar backbone structure. This would result in a morereproducible membrane. In particular, it is desired that one would beable to predict the resulting glucose permeability of the resultingmembrane by simply varying the polymer composition. In this way, theglucose diffusion characteristics of the membrane could be modified,without greatly changing the manufacturing parameters for the membrane.In particular, there is a need for homogeneous membranes havinghydrophilic segments dispersed throughout a hydrophobic matrix that areeasy to fabricate reproducibly from readily available reagents. Ofparticular importance would be the development of membranes where thehydrophilic portions were distributed evenly throughout the membrane,and where their size and dimensions were on an order considerably lessthan the size and dimensions of the electrode of the sensing device toallow the electrode to be in association with a useful amount of bothhydrophobic and hydrophilic portions. The ability of the membranes to besynthesized and manufactured in reasonable quantities and at reasonableprices would be a further advantage.

SUMMARY OF THE INVENTION

The present invention provides an implantable membrane for controllingthe diffusion of an analyte therethrough to a biosensor with which it isassociated. In particular, the membrane of the present inventionsatisfies a need in the art by providing a homogenous membrane with bothhydrophilic and hydrophobic regions to control the diffusion of glucoseand oxygen to a biosensor, the membrane being fabricated easily andreproducibly from commercially available materials.

The invention provides a biocompatible membrane that regulates thetransport of analytes that includes: (a) a matrix including a firstpolymer; and (b) a second polymer dispersed throughout the matrix,wherein the second polymer forms a network of microdomains which whenhydrated are not observable using photomicroscopy at 400× magnificationor less.

Further provided by the invention is a polymeric membrane for regulationof glucose and oxygen in a subcutaneous glucose measuring device thatincludes: (a) a matrix including a first polymer; and (b) a secondpolymer dispersed throughout the matrix, wherein the second polymerforms a network of microdomains which are not photomicroscopicallyobservable when hydrated at 400× magnification or less.

Yet another aspect of the present invention is directed to a polymericmembrane for regulating the transport of analytes, the membraneincluding at least one block copolymer AB, wherein B forms a network ofmicrodomains which are not photomicroscopically observable when hydratedat 400× magnification or less.

Also provided is a membrane and sensor combination, the sensor beingadapted for evaluating an analyte within a body fluid, the membranehaving: (a) a matrix including a first polymer; and (b) a second polymerdispersed throughout the matrix, wherein the second polymer forms anetwork of microdomains which are not photomicroscopically observablewhen hydrated at 400× magnification or less.

The invention further provides an implantable device for measuring ananalyte in a hydrophilic body fluid, including: (a) a polymeric membranehaving (i) a matrix including a first polymer; and (ii) a second polymerdispersed throughout the matrix, wherein the second polymer forms anetwork of microdomains which are not photomicroscopically observablewhen hydrated at 400× magnification or less; and (b) a proximal layer ofenzyme reactive with the analyte.

Moreover, a method for preparing an implantable membrane according tothe invention is provided, the method including the steps of: (a)forming a composition including a dispersion of a second polymer withina matrix of a first polymer, the dispersion forming a network ofmicrodomains which are not photomicroscopically observable when hydratedat 400× magnification or less; (b) maintaining the composition at atemperature sufficient to maintain the first polymer and the secondpolymer substantially soluble; (c) applying the composition at thistemperature to a substrate to form a film thereon; and (d) permittingthe resultant film to dry to form the membrane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photomicrograph of a cross-section of prior art membrane at200× magnification following hydration with water for two hours.

FIG. 2A is a schematic representation of a cross-section of a prior artmembrane having large hydrated structures dispersed substantiallythroughout a hydrophobic matrix, the hydrated structures beingphotomicroscopically observable at 400× magnification or less. Thefigure illustrates the positioning of a working electrode relative to aglucose diffusion pathway.

FIG. 2B is another schematic representation of a cross-section of theprior art membrane of FIG. 2A, where the working electrode is placed inassociation with a locally high concentration of the hydrophobic matrix.

FIG. 2C is yet another schematic representation of a cross-section ofthe prior art membrane of FIG. 2A where glucose diffusion is variableacross the dimension adjacent the electrode surface.

FIG. 3 is a photomicrograph of a cross-section of a membrane of thepresent invention at 200× magnification following hydration with waterfor two hours.

FIG. 4 is a schematic representation of a cross-section illustrating oneparticular form of the membrane of the present invention that shows anetwork of microdomains which are not photomicroscopically observable at400× or less magnification dispersed through a hydrophobic matrix, wherethe membrane is positioned in association with a sensor that includes aworking electrode.

FIG. 5 is a schematic representation of a cross-section of the membraneof FIG. 3 in combination with an enzyme containing layer positioned moreadjacent to a sensor 50.

FIG. 6 is a graph showing sensor output versus the percent of thehydrophobic-hydrophilic copolymer component in the coating blend.

FIG. 7 is a graph showing the percent standard deviation of the sensorcurrent versus the percent of the hydrophobic-hydrophilic copolymercomponent in the coating blend.

DETAILED WRITTEN DESCRIPTION

In order to facilitate understanding of the present invention, a numberof terms are defined below.

The term “analyte” refers to a substance or chemical constituent in abiological fluid (e.g. blood or urine) that is intended to be analyzed.A preferred analyte for measurement by analyte detecting devicesincluding the membrane of the present invention is glucose.

The term “sensor” refers to the component or region of a device by whichan analyte can be evaluated.

By the terms “evaluated”, “monitored”, “analyzed”, and the like, it ismeant that an analyte may be detected and/or measured.

The phrase “continuous glucose sensing” refers to the period in whichmonitoring of plasma glucose concentration is repeatedly performed overshort periods of time, for example, 10 seconds to about every 15minutes.

The term “domain” refers to regions of the membrane of the presentinvention that may be layers, uniform or non-uniform gradients (e.g.anisotropic) or provided as portions of the membrane. Furthermore, theregion possesses physical properties distinctly different from otherportions of the membrane.

The terms “accurate” and “accurately” means, for example, 85% ofmeasured glucose values are within the “A” and “B” region of a standardClarke Error Grid when the sensor measurements are compared to astandard reference measurement. It is understood that like anyanalytical device, calibration, calibration validation and recalibrationare required for the most accurate operation of the device.

The term “host” refers to humans and other animals.

In the disclosure that follows, the invention will primarily be referredto in terms of assay of glucose and solutions such as blood that tend tocontain a large excess of glucose over oxygen. However, it is wellwithin the contemplation of the present invention that the membrane isnot limited solely to the assay of glucose in a biological fluid, butmay be used for the assay of other compounds. In addition, the sensorprimarily referred to is an electrochemical sensor that directlymeasures hydrogen peroxide. However, it is well within the contemplationof the present invention that non-electrochemical based sensors that useoptical detectors or other suitable detectors may be used to evaluate ananalyte.

Membranes of the prior art have generally been unreliable at limitingthe passage of glucose to implantable glucose sensors. This haspresented a problem in the past in that the amount of glucose cominginto contact with the immobilized enzyme exceeds the amount of oxygenavailable. As a result, the oxygen concentration is the rate-limitingcomponent of the reaction, rather than the glucose concentration, suchthat the accuracy of the glucose measurement in the body fluid iscompromised.

As described above, in contrast to the present invention, a disadvantageof prior art membranes for regulating analyte transport therethrough hasbeen their tendency to form large undesirable structures (see FIG. 1)that are observable when the membrane is hydrated. In particular, thesehydrated structures can be detected by photomicroscopy undermagnifications in the range of between 200×-400×, for example. They havebeen shown by the present inventors to be non-uniform in theirdimensions through the membrane, with some being of the same size andsame order of dimensions as the electrode size. These large structureshave been found to be problematic in that they can result in a locallyhigh concentration of either hydrophobic or hydrophilic material inassociation with the working electrode, which can lead to inaccurateglucose readings. Moreover, they can greatly reduce the number ofglucose diffusion path available.

The membrane of the present invention seeks to circumvent these problemsassociated with prior art membranes by providing a reliable homogeneousmembrane that regulates the transport of glucose or other analytestherethrough, the membrane having (a) a matrix including a firstpolymer; and (b) a second polymer dispersed throughout the matrix,wherein the second polymer forms a network of microdomains which whenhydrated are not observable using photomicroscopy at 400× magnificationor less. In one embodiment of the invention, the membrane issubstantially free of observable domains.

We refer now to FIG. 3, which shows a photomicrograph of a cross-sectionof a membrane 5 according to the present invention following hydrationat two hours. As shown in FIG. 3, the membrane is devoid of anyundesirable, large elliptical or spherical structures, such as wereobservable in hydrated prior art membranes at similar magnifications. Itis noted that particles 6 in membrane 5 are dust particles.

For purposes of the present invention, it is likely that glucosepermeability and diffusion is related to the ratio of hydrophobic tohydrophilic constituents and their distribution throughout the membrane,with diffusion occurring substantially along assembled hydrophilicsegments from the side of the membrane in contact with the host to thesensing side.

Referring now to FIG. 4, membrane 42 of the present invention, inaccordance with a particular arrangement, is schematically shown havinghydrophilic segments 44 dispersed substantially throughout a hydrophobicmatrix 46 and presenting a surface 48 to a hydrophilic body fluid. Thehydrophilic body fluid contains the sample to be assayed. In oneembodiment, the body fluid contains both glucose and oxygen. Membrane 42restricts the rate at which glucose enters and passes through themembrane and/or may increase the rate at which oxygen enters and passesthrough membrane 42.

While not wishing to be bound by any one theory, it is likely thatglucose diffuses substantially along hydrophilic segments 44, but isgenerally excluded from the hydrophobic matrix 46. It is noted thatwhile the hydrophilic segments 44 are shown as comprising discretemicrodomains in FIG. 4, small amounts of hydrophobic polymer may bepresent therein, particularly at the interface with the hydrophobicmatrix 46. Similarly, small amounts of hydrophilic polymer may bepresent in the hydrophobic matrix 46, particularly at the interface withhydrophilic segments 44.

In the embodiment shown in FIG. 4, inventive membrane 42 is shown incombination with a sensor 50, which is positioned adjacent to themembrane. It is noted that additional membranes or layers may besituated between membrane 42 and sensor 50, as will be discussed infurther detail below. Diffusion of the sample along paths 52 throughmembrane 42 into association with a working electrode 54 of sensor 50causes development of a signal that is proportional to the amount ofanalyte in the sample. Determination of the analyte may be made bycalculations based upon similar measurements made on standard solutionscontaining known concentrations of the analyte. For example, one or moreelectrodes may be used to detect the amount of analyte in the sample andconvert that information into a signal; the signal may then betransmitted to electronic circuitry required to process biologicalinformation obtained from the host. U.S. Pat. Nos. 4,757,022, 5,497,772and 4,787,398 describe suitable electronic circuitry that may beutilized with implantable devices of the present invention.

The present invention solves a need in the art by providing a reliablemembrane for controlling glucose diffusion therethrough. As shown inFIG. 4, glucose can traverse along hydrophilic segments 44 from the side48 of the membrane in contact with a body fluid to the side 56 proximalto sensor 50. The hydrophilic microdomains 44 are likely distributedsubstantially evenly throughout the membrane. Furthermore, thesemicrodomains are likely substantially uniform in size throughout themembrane. The size and order to dimensions of these microdomains isconsiderably less than the that of the working electrode 54 of sensor50. As such, the electrode is in association with a useful amount ofboth the hydrophobic 46 and hydrophilic 44 regions of the membrane toallow effective control over the amount of glucose diffusing to theelectrode. Moreover, as shown in FIG. 4, the number of paths availablefor glucose to permeate the membrane and diffuse from side 48 to thesensing side 56 would be greater for the inventive membrane than forprior art membranes. Consequently, more accurate and reproducibleglucose readings are attainable across the entire inventive membrane.

FIG. 5 shows a preferred embodiment of the present invention whereinmembrane 42 is used in combination with a proximal membrane layer 58that comprises an enzyme that is reactive with the analyte. In thisinstance, diffusion of the sample from side 48 through the membrane 42into contact with the immobilized enzyme in layer 58 leads to anenzymatic reaction in which the reaction products may be measured. Forexample, in one embodiment the analyte is glucose. In a furtherembodiment, the enzyme immobilized in layer 58 is glucose oxidase.

As described above, glucose oxidase catalyzes the conversion of oxygenand glucose to hydrogen peroxide and gluconic acid. Because for eachglucose molecule metabolized, there is proportional change in theco-reactant O₂ and the product H₂O₂, one can monitor the change ineither the co-reactant or the product to determine glucoseconcentration. With further reference to FIG. 5, diffusion of theresulting hydrogen peroxide through layer 58 to the sensor 50, (e.g.electrochemically reactive surfaces), causes the development of anelectrical current that can be detected. This enables determination ofthe glucose by calculations based upon similar measurements made onstandard solutions containing known concentrations of glucose.

In addition to glucose oxidase, the present invention contemplates theuse of a layer impregnated with other oxidases, e.g. galactose oxidaseor uricase. For an enzyme-based electrochemical glucose sensor toperform well, the sensor's response must neither be limited by enzymeactivity nor cofactor concentration. Because enzymes, including glucoseoxidase, are subject to deactivation as a function of ambientconditions, this behavior needs to be accounted for in constructingsensors for long-term use.

When the membrane of the present invention is combined with an enzymelayer 58 as shown in FIG. 5, it is the enzyme layer that is located moreproximally to the sensor 50 (e.g. electrochemically reactive surfaces).It is noted that enzyme-containing layer 58 must be of sufficientpermeability to 1) freely pass glucose to active enzyme and 2) to permitthe rapid passage of hydrogen peroxide to the sensor (electrodesurface). A failure to permit the rapid passage of glucose to the activeenzyme or hydrogen peroxide from the active enzyme to the electrodesurface can cause a time delay in the measured signal and thereby leadto inaccurate results.

Preferably, the enzyme layer is comprised of aqueous polyurethane-basedlatex into which the enzyme is immobilized.

It is noted that while the inventive membrane 42 may itself containimmobilized enzymes for promoting a reaction between glucose and oxygen,it is preferred that the enzyme be located in a separate layer, such aslayer 58 shown in FIG. 5. As described above, it is known that enzymeactively reacting with glucose is more susceptible to irreversibleinactivation. Therefore, a disadvantage of providing enzyme in a layerthat is semi-permeable to glucose, is that the calibration factors ofthe sensor may change over time as the working enzyme degrades. Incontrast, when enzyme is dispersed throughout a membrane freelypermeable to glucose (i.e. layer 58 in FIG. 5), such a membrane islikely to yield calibration factors that are more stable over the lifeof a sensor.

In one preferred embodiment of the invention, the first polymer of themembrane includes homopolymer A and the second polymer includescopolymer AB.

In another embodiment, the first polymer includes copolymer AB and thesecond polymer includes copolymer AB. Preferably, the amount of B incopolymer AB of the first polymer is different than the amount of B incopolymer AB of the second polymer. In particular, the membrane may beformed from a blend of two AB copolymers, where one of the copolymerscontains more of a hydrophilic B polymer component than the blendedtargeted amount and the other copolymer contains less of a hydrophilic Bpolymer component than the blended targeted amount.

In yet another embodiment of the invention, the first polymer includeshomopolymer A and the second polymer includes homopolymer B.

As described above, the invention also provides a polymeric membrane forregulating the transport of analytes that includes at least one blockcopolymer AB, wherein B forms a network of microdomains which are notphotomicroscopically observable when hydrated at 400× magnification orless. In one embodiment, the ratio of A to B in copolymer AB is 70:30 to90:10.

For each of the inventive embodiments herein described, homopolymer A ispreferably a hydrophobic A polymer. Moreover, copolymer AB is preferablya hydrophobic-hydrophilic copolymer component that includes the reactionproducts of a hydrophobic A polymer and a hydrophilic B polymer.Suitable materials for preparing membranes the present invention aredescribed below.

For purposes of the present invention, copolymer AB may be a random orordered block copolymer. Specifically, the random or ordered blockcopolymer may be selected from the following: ABA block copolymer, BABblock copolymer, AB random alternating block copolymer, AB regularlyalternating block copolymer and combinations thereof.

In a preferred embodiment, the sensor, membrane, and methods of thepresent invention may be used to determine the level of glucose or otheranalytes in a host. The level of glucose is a particularly importantmeasurement for individuals having diabetes in that effective treatmentdepends on the accuracy of this measurement.

In particular, the invention provides a method of measuring glucose in abiological fluid that includes the steps of: (a) providing (i) a host,and (ii) an implantable device for measuring an analyte in a hydrophilicbody fluid, where the device includes a polymeric membrane having amatrix including a first polymer and a second polymer dispersedthroughout the matrix, wherein the second polymer forms a network ofmicrodomains which are not photomicroscopically observable when hydratedat 400× magnification or less; and a proximal layer of enzyme reactivewith the analyte; and (b) implanting the device in the host. In oneembodiment, the device is implanted subcutaneously.

The invention also provides a method of measuring glucose in abiological fluid that includes the following steps: (a) providing (i) ahost, and (ii) an implantable device for measuring an analyte in ahydrophilic body fluid, that includes a polymeric membrane including amatrix including a first polymer and a second polymer dispersedthroughout the matrix, wherein the second polymer forms a network ofmicrodomains which are not photomicroscopically observable when hydratedat 400× magnification or less; and a proximal layer of enzyme reactivewith the analyte, the device being capable of accurate continuousglucose sensing; and (b) implanting the device in the host. Desirably,the implant is placed subcutaneously in the host.

Glucose sensors that use, for example, glucose oxidase to effect areaction of glucose and oxygen are known in the art, and are within theskill of one in the art to fabricate (see, for example, U.S. Pat. Nos.5,165,407, 4,890,620, 5,390,671, 5,391,250, 6,001,067 as well ascopending, commonly owned U.S. patent application Ser. No. 09/916,858.It is noted that the present invention does not depend on a particularconfiguration of the sensor, but is rather dependent on the use of theinventive membrane to cover or encapsulate the sensor elements.

For the electrochemical glucose sensor to provide useful results, theglucose concentration, as opposed to oxygen concentration, must be thelimiting factor. In order to make the system sensitive to glucoseconcentration, oxygen must be present within the membrane in excess ofthe glucose. In addition, the oxygen must be in sufficient excess sothat it is also available for electrochemical reactions occurring at theamperometric electrode surfaces. In a preferred embodiment, theinventive membrane is designed so that oxygen can pass readily into andthrough the membrane and so that a reduced amount of glucose diffusesinto and through the membrane into contact with an immobilized glucoseoxidase enzyme. The inventive membrane allows the ratio of oxygen toglucose to be changed from a concentration ratio in the body fluid ofabout approximately 50 and 100 parts of glucose to 1 of oxygen to a newratio in which there is a stoichiometric excess of oxygen in the enzymelayer. Through the use of the inventive membrane, an implantable glucosesensor system is not limited by the concentration of oxygen present insubcutaneous tissues and can therefore operate under the premise thatthe glucose oxidase reaction behaves as a 1-substrate (glucose)dependent process.

The present invention provides a semi-permeable membrane that controlsthe flux of oxygen and glucose to an underlying enzyme layer, renderingthe necessary supply of oxygen in non-rate-limiting excess. As a result,the upper limit of linearity of glucose measurement is extended to amuch higher value than that which could be achieved without the membraneof the present invention. In particular, in one embodiment the membraneof the present invention is a polymer membrane with oxygen-to-glucosepermeability ratios of approximately 200:1; as a result, 1-dimensionalreactant diffusion is adequate to provide excess oxygen at allreasonable glucose and oxygen concentrations found in a subcutaneousmatrix [Rhodes, et al., Anal. Chem., 66: 1520-1529 (1994)].

A hydrophilic or “water loving” solute such as glucose is readilypartitioned into a hydrophilic material, but is generally excluded froma hydrophobic material. However, oxygen can be soluble in bothhydrophilic and hydrophobic materials. These factors affect entry andtransport of components in the inventive membrane. The hydrophobicportions of the inventive membrane hinder the rate of entry of glucoseinto the membrane, and therefore to the proximal enzyme layer whileproviding access of oxygen through both the hydrophilic and hydrophobicportions to the underlying enzyme.

In one preferred embodiment, the membrane of the invention is formedfrom a blend of polymers including (i) a hydrophobic A polymercomponent; and (ii) a hydrophobic-hydrophilic copolymer componentblended with component (i) that forms hydrophilic B domains that controlthe diffusion of an analyte therethrough, wherein the copolymercomponent includes a random or ordered block copolymer. Suitable blockcopolymers are described above. One is able to modify the glucosepermeability and the glucose diffusion characteristics of the membraneby simply varying the polymer composition.

In one preferred embodiment, the hydrophobic A polymer is apolyurethane. In a most preferred embodiment, the polyurethane ispolyetherurethaneurea. A polyurethane is a polymer produced by thecondensation reaction of a diisocyanate and a difunctionalhydroxyl-containing material. A polyurethaneurea is a polymer producedby the condensation reaction of a diisocyanate and a difunctionalamine-containing material. Preferred diisocyanates include aliphaticdiisocyanates containing from 4 to 8 methylene units. Diisocyanatescontaining cycloaliphatic moieties, may also be useful in thepreparation of the polymer and copolymer components of the membrane ofthe present invention. The invention is not limited to the use ofpolyurethanes as the hydrophobic polymer A component. The material thatforms the basis of the hydrophobic matrix of the inventive membrane maybe any of those known in the art as appropriate for use as membranes insensor devices and having sufficient permeability to allow relevantcompounds to pass through it, for example, to allow an oxygen moleculeto pass through the inventive membrane from the sample under examinationin order to reach the active enzyme or electrochemical electrodes.Examples of materials which may be used to make a non-polyurethane typemembrane include vinyl polymers, polyethers, polyesters, polyamides,inorganic polymers such as polysiloxanes and polycarbosiloxanes, naturalpolymers such as cellulosic and protein based materials and mixtures orcombinations thereof.

As described above, the hydrophobic-hydrophilic copolymer componentincludes the reaction products of a hydrophobic A polymer component anda hydrophilic B polymer component. The hydrophilic B polymer componentis desirably polyethylene oxide. For example, one usefulhydrophobic-hydrophilic copolymer component is a polyurethane polymerthat includes about 20% hydrophilic polyethyelene oxide. Thepolyethylene oxide portion of the copolymer is thermodynamically drivento separate from the hydrophobic portions of the copolymer and thehydrophobic A polymer component. The 20% polyethylene oxide based softsegment portion of the copolymer used to form the final blend controlsthe water pick-up and subsequent glucose permeability of the membrane ofthe present invention.

The polyethylene oxide may have an average molecular weight of from 200to 3000 with a preferred molecular weight range of 600 to 1500 andpreferably constitutes about 20% by weight of the copolymer componentused to form the membrane of the present invention.

It is desired that the membrane of the present invention have athickness of about 5 to about 100 microns. In preferred embodiments, themembrane of the present invention is constructed of apolyetherurethaneurea/polyetherurethaneurea-block-polyethylene glycolblend and has a thickness of not more than about 100 microns, morepreferably not less than about 10 microns, and not more than about 80microns, and most preferably, not less than about 20 microns, and notmore than about 60 microns.

The membrane of the present invention can be made by casting fromsolutions, optionally with inclusion of additives to modify theproperties and the resulting cast film or to facilitate the castingprocess.

The present invention provides a method for preparing the implantablemembrane of the invention. The method includes the steps of: (a) forminga composition including a dispersion of a second polymer within a matrixof a first polymer, the dispersion forming a network of microdomainswhich are not photomicroscopically observable when hydrated at 400×magnification or less; (b) maintaining the composition at a temperaturesufficient to maintain the first polymer and the second polymersubstantially soluble; (c) applying the composition at the temperatureto a substrate to form a film thereon; and (d) permitting the resultantfilm to dry to form the membrane. In one embodiment, the forming stepincludes forming a mixture or a blend. As described above, in preferredembodiments, the first polymer is a polyurethane and the second polymeris polyethylene oxide. In general, the second polymer may be a random orordered block copolymer selected from the following: ABA blockcopolymer, BAB block copolymer, AB random alternating block copolymer,AB regularly alternating block copolymer and combinations thereof.

In one embodiment, the composition comprised of a dispersion of thesecond polymer within the matrix of a first polymer is heated to atemperature of about 70° C. to maintain the first and second polymerssubstantially soluble. For example, the combination of a hydrophobicpolymer A component and a hydrophobic-hydrophilic copolymer AB componentis desirably exposed to a temperature of about 70° C. to maintain thepolymer and copolymers substantially soluble. In particular, the blendis heated well above room temperature in order to keep the hydrophilicand hydrophobic components soluble with each other and the solvent.

The invention contemplates permitting the coated film formed on thesubstrate to dry at a temperature from about 120° C. to about 150° C.The elevated temperature further serves to drive the solvent from thecoating as quickly as possible. This inhibits the hydrophilic andhydrophobic portions of the membrane from segregating and forming largeundesired structures.

The membrane and sensor combinations of the present invention provide asignificant advantage over the prior art in that they provide accuratesensor operation at temperatures from about 30° C. to about 45° C. for aperiod of time exceeding about 30 days to exceeding about a year.

EXAMPLES Example 1 A Method for Preparing a Membrane of the PresentInvention

The inventive membrane may be cast from a coating solution. The coatingsolution is prepared by placing approximately 281 gm ofdimethylacetamide (DMAC) into a 3 L stainless steel bowl to which asolution of polyetherurethaneurea (344 gm of Chronothane H (CardiotechInternational, Inc., Woburn, Mass.), 29,750 cp @ 25% solids in DMAC) isadded. To this mixture is added another polyetherurethaneurea(approximately 312 gm, Chronothane 1020 (Cardiotech International, Inc.,Woburn, Mass.), 6275 cp @ 25% solids in DMAC). The bowl is then fittedto a planetary mixer with a paddle-type blade and the contents arestirred for 30 minutes at room temperature. Coatings solutions preparedin this manner are then coated at between room temperature to about 70°C. onto a PET release liner (Douglas Hansen Co., Inc., Minneapolis,Minn.) using a knife-over-roll set at a 0.012 inch gap. The film iscontinuously dried at 120° C. to about 150° C. The final film thicknessis approximately 0.0015 inches.

Observations of Membrane Using Photomicroscopy at 400× Magnification orLess

A ¼″ by ¼″ piece of membrane is first immersed in deionized water for aminimum of 2 hours at room temperature. After this time, the sample isplaced onto a microscope slide along with one drop of water. A glasscover slide is then placed over the membrane and gentle pressure isapplied in order to remove excess liquid from underneath the coverglass. In this way, the membrane does not dry during its evaluation. Thehydrated membrane sample is first observed at 40×-magnification using alight microscope (Nikon Eclipse E400). If air bubbles are present on thetop or bottom of the film, the cover glass is gently pressed again witha tissue in order to remove them. Magnification is then increased to200×; and the hydrated membrane is continuously observed while changingthe focus from the top to bottom of the film. This is followed by anincrease in magnification to 400×, with the membrane again beingcontinuously observed while changing the focus from the top to bottom ofthe film.

Results

Based on the results of an optical micrograph of a sample membraneprepared by using a room temperature coating solution and drying of thecoated film at 120° C., the micrograph being captured as describedabove, it was noticed that both circular and elliptical domains werepresent throughout the hydrated section of membrane. At the samemagnification, the domains were not observable in dry membrane. Givingthat in an electrochemical sensor, the electrodes included therein aretypically of the same size and same order of dimensions as the observedcircular and elliptical domains, such domains are not desired. Thesedomains present a problem in that they result in a locally highconcentration of either hydrophilic or hydrophobic material inassociation with the electrodes.

Example 2 Optimizing the Coating Solution Conditions

This example demonstrates that preheating the coating solution totemperature of 70° C. prior to coating eliminates the presence of boththe circular and elliptical domains that were present throughout thehydrated cross-section of a membrane prepared using a room temperaturecoating solution and drying of the coated film at 120° C. Example 2further demonstrates that, provided the coating solution is preheated toabout 70° C., either a standard (120°) or elevated (150° C.) dryingtemperature were sufficient to drive the DMAC solvent from the coatedfilm quickly to further inhibit the hydrophilic and hydrophobic portionsof the polyurethane membrane from segregating into large domains.

In particular, the invention was evaluated by performing a coatingexperiment where standard coating conditions (room temperature coatingsolution and 120° C. drying temperature of the coated film) werecompared to conditions where the coating solution temperature waselevated and/or the drying temperature of the coated film was elevated.Four experimental conditions were run as follows:

SS-room temperature solution and standard (120° C.) oven temperature.

SE-room temperature solution and elevated (150° C.) oven temperature.

ES-preheated (70° C.) solution and standard (120° C.) oven temperature.

EE-preheated (70° C.) solution and elevated (150° C.) oven temperature.

Results

Samples of each of the four membranes listed above were then hydratedfor 2 hours, and then observed under the microscope. Performancespecifications were achieved when the micrograph of the membraneprepared under a given condition showed an absence of circular and/orelliptical domains that result in an undesirable, discontinuoushydrophilic and hydrophobic membrane structure. Table 1 below summarizesthese results where (+) indicates a membrane meeting desired performancespecifications and (−) is indicative of a membrane showing theundesirable circular and/or elliptical domains. In summary, for both theES and EE conditions, where the coating solution was preheated to 70° C.prior to coating on a substrate, no hydrated domains were observed at a200× magnification. Furthermore, regardless of the drying temperatureused for the coated film, when the coating solution was not preheated(conditions SS and SE), the hydrated structures were observed.Therefore, it is likely that preheating the coating solution effectivelyinhibits the hydrophilic and hydrophobic segments of the polyurethanefrom segregating into large domains.

TABLE 1 Coating Condition Result SS − SE − ES (Inventive) + EE(Inventive) +

Example 3 Evaluation of the Inventive Membranes for their Permeabilityto Glucose and H₂O₂

Membranes prepared under the EE condition described in Example 2 wereevaluated for their ability to allow glucose and hydrogen peroxide toget through the membrane to a sensor. In particular, a series ofpolyurethane blends of the present invention were generated wherein thepercentage of Chronothane H in a coating blend was varied. Furthermore,one of these blends (57.5% Chronothane H in coating blend) was preparedunder both the EE condition and the SS condition as described in Example2. FIG. 6 shows that the sensor output generated with a series ofpolyurethane blends of the present invention was dependent upon thepercentage of the Chronothane H. In particular, the sensor outputincreased as the percentage of Chronothane H in the coating blendincreased. With further reference to FIG. 6, when the percentage ofChronothane H in the coating blend was 57.5%, the sensor output wasthree times greater for the membrane prepared under the optimized EEcoating condition as compared to the non-optimized SS coating condition.

Furthermore, FIG. 7 demonstrates that, regardless of the percentChronothane H in the coating blend, an inventive membrane prepared underthe EE condition shows a fairly constant percent standard deviation ofsensor output. Moreover, a membrane prepared with 57.5% Chronothane H inthe coating blend under the SS condition showed a percent standarddeviation of sensor output approximately twice that of an EE membraneprepared with the same percentage of Chronothane H in the blend. It isnoted that given that the sensor output is a true measure of the amountof glucose getting through the membrane to the sensor, the resultsindicate that the permeability of glucose and H₂O₂ is relativelyconstant throughout a given inventive membrane prepared under optimizedcoating conditions (i.e., EE conditions). This is important from amanufacturing standpoint.

Having described the particular, preferred embodiments of the inventionherein, it should be appreciated that modifications may be madetherethrough without departing from the contemplated scope of theinvention. The true scope of the invention is set forth in the claimsappended hereto.

1. An implantable electrochemical sensor for measuring glucose in abodily fluid, the sensor comprising: at least one electrode having atleast a portion configured to be implanted in a host; and a membranedisposed over the at least one electrode, the membrane comprising: afirst layer comprising a blend of (i) a hydrophobic polymer that forms ahydrophobic matrix and (ii) a hydrophobic-hydrophilic copolymercomprising hydrophilic polymer segments that traverse a thickness of thefirst layer, wherein the first layer is further configured to reduce aflux of glucose through the membrane, wherein the hydrophobic polymerand the hydrophobic-hydrophilic copolymer are soluble with each other,and wherein the hydrophilic polymer segments comprise hydrophilicmicrodomains that are substantially distributed evenly throughout thehydrophobic matrix; and a second layer comprising an enzyme capable ofreacting with glucose, wherein the second layer is more proximal to theelectrode than the first layer.
 2. The implantable electrochemicalsensor of claim 1, wherein a hydrophobic component of thehydrophobic-hydrophilic copolymer and a hydrophobic component of thehydrophobic polymer comprise a same backbone structure.
 3. Theelectrochemical sensor of claim 1, wherein the membrane is configured toincrease a rate at which oxygen enters and passes through the membrane.4. The electrochemical sensor of claim 1, wherein the enzyme is anoxidase.
 5. The electrochemical sensor of claim 1, wherein the enzyme isglucose oxidase.
 6. The electrochemical sensor of claim 1, wherein theelectrode is platinum.
 7. The electrochemical sensor of claim 1, whereinthe sensor is configured such that a current measured by the sensor isgenerated by oxidation of hydrogen peroxide at the electrode.
 8. Theelectrochemical sensor of claim 1, wherein the membrane is configured tochange a ratio of oxygen to glucose from a concentration ratio in thebodily fluid of about approximately 50 to 100 parts of glucose to 1 partof oxygen to a ratio in which there is a stoichiometric excess of oxygenin the second layer.
 9. The electrochemical sensor of claim 1, whereinthe membrane has an oxygen-to-glucose permeability ratio ofapproximately 200:1.
 10. The electrochemical sensor of claim 1, whereinthe first layer has a thickness of from about 5 to about 100 microns.11. The electrochemical sensor of claim 1, wherein the hydrophilicpolymer segments comprise a plurality of hydrophilic microdomains thatare substantially uniform in size throughout the first layer.
 12. Theelectrochemical sensor of claim 1, wherein the hydrophilic polymersegments are dispersed substantially throughout the hydrophobic matrix.13. The electrochemical sensor of claim 1, wherein the hydrophilicpolymer segments comprise a network of microdomains which are notphotomicroscopically observable when hydrated at 400× magnification orless.
 14. The electrochemical sensor of claim 1, wherein theelectrochemical sensor during use is capable of obtaining an accuracylevel corresponding to having 85% of measured glucose values beingwithin A and B regions of a standard Clarke Error Grid when the measuredglucose values are compared to standard reference measurements.
 15. Theelectrochemical sensor of claim 1, wherein the hydrophobic matrixcomprises polyurethane.
 16. The electrochemical sensor of claim 15,wherein the polyurethane comprises polyetherurethaneurea.
 17. Theelectrochemical sensor of claim 1, wherein the hydrophilic polymersegments comprise polyethylene oxide.
 18. The electrochemical sensor ofclaim 17, wherein the polyethylene oxide has an average molecular weightof from 200 to
 3000. 19. The electrochemical sensor of claim 17, whereinthe polyethylene oxide has an average molecular weight of from 600 to1500.